Semiconductors such as transistors and diodes used in electronic equipments, such as converters and electrical power sources, need to have high performance, high speed, small size and high integration and, accordingly, generate a large amount of heat. The heat raises a temperature of the equipments and causes a malfunction or destruction. Therefore, many heat dissipation methods and heat dissipation materials to be used in the method have been proposed to be in order to suppress the temperature raise of the semiconductors in operation.
For example, in order to suppress the temperature raise of semiconductors in operation of electronic equipments, a heat sink having a metal plate which has a high thermal conductivity, such as aluminum or copper plates, is used. This heat sink transmits the heat generated in a semiconductor and releases the heat from the surface on account of a difference in a temperature from outside air. Meanwhile, the semiconductor must be electrically isolated from the heat sink and, therefore, a plastic film is interposed between the heat-generating electronic part and the heat sink. However, the thermal conductivity of the plastic film is very low, so that the heat transfer to the heat sink is significantly prevented. In a case where a semiconductor such as a transistor is fixed to the heat sink with a screw, the screw must pierce through the plastic film to cause a hole in the film. Then, the film tears at the hole to loose the insulation property. The lost insulation property is fatal for transistors and diodes.
In order to provide a sheet which has a good thermally conductivity and less tear, a thermally conductive sheet has been developed wherein a glass cloth and a thermally conductive resin are laminated. For instance, Patent Literature 1 describes a thermally conductive sheet wherein silicone rubber comprising boron nitride powder and spherical silica powder as thermally conductive filler is laminated on a glass cloth. A pressure is applied in the preparation of the sheet. To this end, it is necessary to prepare a uncured sheet having an appropriate size and use a press molding machine. Further, the preparation is in a batch manufacture and, therefore, it is impossible to roll up a finished thermally conductive sheet. This is very inefficient in terms of the productivity and the yield. Further, a size of the raw sheet is restricted, so that a molding size is also restricted.
Coating molding is known as a method for continuous molding. For example, one side of a glass cloth is filled with a thermally conductive silicone resin and, then, a thermally conductive silicone rubber layer is formed on the filled glass cloth to continuously prepare a thermally conductive sheet. The coating molding is very efficient because the finished sheet can be continuously rolled up. In addition, although the width of the sheet is restricted depending on a size of a coating apparatus, the length of the sheet is not restricted and the degree of freedom in size in molding is very high, compared to that in the press molding. However, flatness of the surface obtained in the coating molding is poor, compared to that in the press molding, so that a contact thermal resistance is high. Further, a density of the silicone rubber layer is low, because no pressure is applied in the coating molding. Therefore, the coating molding is not suitable as a method for preparing a sheet having a high thermal conductivity.